ruby2_keywords is runtime dep not development dep

And so should be included as such in the gemspec; not in the Gemfile.

I've specified the '>= 0.0.5' constraint, because v0.0.5 is the version
we've been testing with.

I've used a '>=' constraint rather than a twiddle-wakka ('~> 0.0.5'),
because that's equivalent to the combination of '>= 0.0.5' and '< 0.1.0'
which seems a bit too restrictive. At the moment there's no reason to
suppose later versions of ruby2_keywords won't work with Mocha and we
can always add a constraint later if it turns out there is a problem.

Closes #581.

Gemfile

@@ -28,5 +28,3 @@ if ENV['MOCHA_GENERATE_DOCS']
   gem 'redcarpet'
   gem 'yard'
 end
-
-gem 'ruby2_keywords', '~> 0.0.5'

mocha.gemspec

@@ -21,4 +21,6 @@ Gem::Specification.new do |s|
   s.homepage = 'https://mocha.jamesmead.org'
   s.require_paths = ['lib']
   s.summary = 'Mocking and stubbing library'
+
+  s.add_runtime_dependency 'ruby2_keywords', '>= 0.0.5'
 end